This invention relates to optical communication systems and, in particular, to an optical communication system which optically combines baseband signals and passband signals and transmits the combined signals over a common optical fiber.
An ever increasing communication need of today is to deliver multimedia services such as voice, data, high speed internet access, video conferencing, video on demand, and broadcast television video to small businesses and residences. Cost is the prominent issue for the deployment of such networks. Among various technologies that are currently available and being deployed, optical fiber extending to usersxe2x80x94Fiber to the Home (FTTH)xe2x80x94is the preferred technology to meet present and future needs. Service providers are taking fiber as deep into their networks as their costs allow.
Two different optical fiber communication systems have evolved for carrying information in digital formats to homes and businesses. One system delivers information by a digitally modulated series of light pulses. These are referred to as baseband signals. A second system uses a plurality of frequency separated carriers. Each carrier is modulated to transmit a digital signal. These are passband signals. Each system has its own specialized equipment, its own physical plant and its own standards.
FIG. 1A schematically illustrates a baseband system 10 comprising a central office 11 providing optical fiber connections to a plurality of homes 12 and businesses 13. High power optical signals at single or multiple wavelengths are transmitted over a plurality of access fibers 15A, 15B, 15C to respective optical power splitters and/or wavelength demultiplexers 16A, 16B, 16C, and at each power splitter or demultiplexer, e.g., 16B, the high power signal is divided into a plurality of lower power or separate wavelength signals and transmitted over a respective plurality of end user fibers 17A and 17B. These signals are called downstream signals. The downstream signals are typically a digitally modulated baseband series of light pulses centered in the 1.3-1.6 xcexcm wavelength band. Signals from the end users to the central office, called upstream signals, are typically digitally modulated baseband pulses in the same 1.3-1.6 wavelength band but at different wavelength from the downstream wavelength. They are transmitted in the reverse direction over the same fibers. The upstream signals can be buffered and time division multiplexed for burst transmission at the power splitters, e.g., 16B. Since this system does not employ any active electronic or photonic component between the central office and the users, it is called a Passive Optical Network (PON).
FIG. 1B illustrates a simplified baseband modulation scheme. Typically, a digital 1 is represented by a light pulse in the series. A digital 0, by the absence of a pulse in a pulse position. Alternatively, the signal can be inverted with a pulse representing digital 0 and its absence representing 1.
FIG. 2A schematically illustrates a passband system 20 comprising a hub 21, and a plurality of fibers 22A, 22B, 22C connecting the hub to a respective plurality of fiber nodes 23A, 23B and 23C. Each node is connected, as by a plurality of fibers or coaxial cables 24A and 24B to a plurality of homes 12 and businesses 13.
FIG. 2B illustrates the radio frequency spectrum of a typical digitally modulated passband signal. The signal comprises a plurality of different radio frequency (RF) carriers spaced apart in frequency (e.g. 6 MHz spacing in the NTSC system). Each of the carriers is modulated among a plurality of states to carry a higher order digital signal to encode plural bits for each modulation state. The modulation can be amplitude modulation, frequency modulation, phase modulation or a combination of them.
Digital passband signals are conventionally transmitted using two RF carriers that are frequency locked but 90 degrees out of phase. The two carriers are said to be in quadrature. The two carriers are separately amplitude modulated (AM), and the modulated carriers are combined to form a single RF output having both amplitude information corresponding to their vector sum and phase information corresponding to their vector angle. The technique is known as quadrature amplitude modulation or QAM.
FIG. 2C illustrates the simplest case of QAM which occurs when each of the carriers has only two states (e.g. +V and xe2x88x92V). One carrier, is considered the reference carrier and is called the in-phase channel. Its amplitude is represented along the horizontal axis of FIG. 2(C). The other carrier, 90xc2x0 out of phase, is called the quadrature channel. Its amplitude is represented along the vertical axis. As can be seen from the diagram, if each carrier has two states (+V, xe2x88x92V), then there are four possible combined outputs, each of which can represent two bits of information: (0,0), (0,1), (1,0), (1,1). This simple modulation scheme is known as quadrature phase shift keying (QPSK).
Similar modulation schemes can be based on amplitude modulation of the carriers among a larger number of states. For example if both carriers can be modulated among four amplitudes, the combined output can represent 4xc3x974=16 states, and the modulation is called 16 QAM modulation. Modulation using 8xc3x978=64 states is 64 QAM. With an increasing number of modulation states, the required signal-to-noise ratio also increases.
In the past few years there has been an international effort from service providers and system manufacturers to define common specifications aimed at the extension of fiber all the way to homes and businesses to deliver existing and future services. These specifications are now part of International Telecommunication Union (ITU) standard G.983.1
According to G.983.1, all services are transported in baseband format in both the upstream and downstream directions on a power splitter-based system. In one variant of the network, a shared 155-Mbps baseband signal is transported downstream in the 1.5-xcexcm band and the same bit rate is sent upstream in the 1.3-xcexcm band on a single fiber. For low cost, a single transmitter in the central office and a single fiber can serve up to 32 users if the fiber is all the way to the user""s premises. The number of users can even be greater if the receiver is at the curb and electrical signals are distributed to multiple dwellings. The G.983.1 specification calls for a minimum logical reach of at least 20 km and an optical power budget consistent with that reach. The specified downstream receiver sensitivity at a bit error ratio of  less than 1010 is xe2x88x9230 dBm for Class B operation and xe2x88x9233 dBm for Class C.
A downstream capacity of 155 Mbps shared among 32 end users is more than adequate for interactive services such as voice, data, or interactive video, but can be quickly exhausted by multichannel broadcast video, especially if high definition TV (HDTV) is to be delivered. One approach to dealing with broadcast video delivery in G983.1 is to increase the downstream bandwidth from 155 to 622 Mbps. This approach is very expensive and complicates video channel switching. Alternatively, video signals can be delivered on a separate fiber using a separate transmitter and a separate receiver. This approach is even more expensive. Accordingly, there is a need for a new approach which improves the performance and lowers the cost.
An optical communication system for gracefully combining both baseband and passband signals on a common fiber is described in applicant""s U.S. patent application Ser. No. 09/432,936 filed Nov. 3, 1999 and entitled xe2x80x9cOptical Communication System Combining Both Baseband and Passband Signalsxe2x80x9d, which is incorporated herein by reference. In this system, the baseband and passband signals are electrically combined, and the combined signal modulates an optical output signal at the Central office. The optical signal can be sent over an optical fiber to a remote power splitter where it is passively power split among a plurality of fibers to respective end users. Within the power budgets of ITU-T G.983.1, this architecture can support the QPSK modulation format that satellite TV uses for class B operation with a PIN diode receiver or class C operation with an APD receiver. For terrestrial transmission of broadcast digital services, most service provider""s, such as providers of cable TV, or wireless cable TV (MMDS services), use 64 QAM or higher order modulation. Compared to QPSK, delivery of 64 QAM modulated signal requires about 13 dB more signal to noise ratio in the electrical domain which means 6.5 dBm more optical power at the receiver. Alternatively, the receiver should be at least 6.5 dB more sensitive. Experimental data show that to deliver a 64 QAM modulated passband signal on top of a 155 Mbps baseband signal in class B of G.983.1, an APD based receiver is required. An APD is much more expensive than a PIN diode. The APD operation requires a supply of typically more than 50V with much more complex voltage and temperature stabilization circuitry as compared to a PIN diode that requires less than 5V and much simpler circuitry.
Telecommunication and Cable TV service providers would prefer to use 64 QAM or higher order modulation for broadcast digital services to make use of their existing video infrastructure and to use the bandwidth efficiency of 64 QAM that can deliver up to 6 bits per Hertz as opposed to a maximum of 2 bits per Hertz by QPSK. To deliver 6.5 dBm more optical power for 64 QAM and to remain within the power budgets of ITU-T G983.1 for the baseband signal, there is a need for a different architecture (system) that can combine baseband and passband signals with an even higher level of performance.
This invention is a new communication system in which multichannel broadcast digital services are distributed to each user with the broadcast services signal riding in the passband above a digital baseband signal. The system can deliver more than 1 Gbps additional bandwidth to each subscriber. The passband bandwidth will accommodate growth in downstream services including video on demand, higher speed web downloads including improved streaming audio and video, HDTV, interactive video, and personalized video. The invention requires only a single fiber path and a single optical receiver for each user or group of users. A single fiber, single optical receiver system is much less expensive than two systems, one transmitting baseband and the other passband. A single receiver is greatly cost beneficial to achieving economical fiber to the home.
Although the result of the invention is to add digital video and other bandwidth demanding services on the system described in G983.1, it can also be used in other architectures where specifications or requirements differ from G983.1. For example, upstream or downstream data rate and optical wavelengths and the required receiver sensitivity and bit error rates may vary according to the specific application. The invention can also be used in point to point transmission of baseband and passband signals on a single fiber and receiver. Furthermore, the optical receiver or ONT does not have to be at the customer premises. It can be outside on the curb. From the curb, the baseband and passband services can be delivered to subscribers sharing that ONT on twisted copper wires or coaxial cables, in an architecture popularly known as Fiber to the Curb (FTTC). For twisted copper wires, the services can be delivered using any of the conventional digital subscriber line techniques. For coaxial lines, hybrid fiber-coaxial (HFC) technology is used.
In an exemplary embodiment, an optical communication system comprises a first optical transmitter for generating an optical baseband signal, a second optical transmitter for generating an optical passband signal, an optical power coupler for combining the signals, a length of optical transmission fiber for transmitting the combined optical signal, and one or more receivers optically coupled to the fiber. In an advantageous system, an optical power splitter is optically coupled to the transmission fiber for power splitting the transmitted signal among a plurality of end-user fibers, and, for each user or group of users, an optical receiver is coupled to the user fiber.